Rhynchotherium is an extinct genus of gomphothere endemic to the southern USA and Mexico from the Miocene to the Early Pleistocene (5.9 to 2.2 Ma). This temporal range also corresponds from the late Hemphillian faunal stage (Hh4) to the Late Blancan faunal stage, with Rhynchotherium being considered by some researchers as a taxon emblematic of the Blancan faunal stage. Rhynchotherium had two tusks and likely evolved from Gomphotherium.
Taxonomy
Rhynchotherium was first described in 1868 on the basis of a lower jaw from the Miocene of Tlaxcala, Mexico.FALCONER, H. (1868): Paleontological Memoirs. Volume II: 74 –75; London (Hardwicke). Later, the type species epithet R. tlascalae was erected for the jaw by Henry Fairfield Osborn in 1918. In 1921, a gomphothere skull from the Mt. Eden area of southern California was described as a subspecies of Trilophodon shepardi (a now-defunct combination for Mastodon shepardi), T. s. edensis,C. Frick. 1921. Extinct vertebrate faunas of the badlands of Bautista Creek and San Timoteo Canyon, southern California. University of California Publications in Geology 12(5):277-424 but was subsequently reassigned to Rhynchotherium.FRICK, C. (1933): New Remains of Trilophodont-Tetralophodont mastodons. – Bulletin American Museum of Natural History, 59: 505 – 652. Other species subsequently assigned to Rhynchotherium included R. falconeri,OSBORN, H. F. (1923): New subfamily, generic and specific stages in the evolution of the proboscidea. – American Museum Novitates, 99: 1– 4. R. paredensis, R. browni,OSBORN, H. F. (1936): Proboscidea: a monograph of the discovery, evolution, migration, and extinction of the mastodonts and elephants of the world, vol. 1: Moeritherioidea, Deinotherioidea, Mastodontoidea. New York (The American Museum Press). and R. simpsoni.OLSEN, S. J. (1957): A new beak-jawed mastodont from Florida. – Journal of the Palaeontological Society of India, 2: 131–135. It was the closest relative to Cuvieronius, and may be ancestral to it.
Lucas and Morgan (2008) reviewed the taxonomy of Rhynchotherium and concluded that only R. edensis, R. falconeri, R. paredensis, R. browni, and R. simpsoni could be confidently referred to Rhynchotherium. Because the genotype of Rhynchotherium is referable to Gomphotherium, the ICZN was petitioned to conserve the genus by designating R. falconeri as the type species,LUCAS, S. G. (2010): Rhynchotherium Falconer, 1868 (Mammalia; Proboscidea): proposed conservation of usage by designation of Rhynchotherium falconeri OSBORN, 1923 as the type species. – Bulletin of Zoological Nomenclature, 67: 158 –162. which it did.
Misassigned species Mastodon shepardi Leidy, 1871 Mastodon euhypodon Cope, 1884 - likely a species of GomphotheriumLUCAS, S.G. & MORGAN, G.S., 2008. Taxonomy of Rhynchotherium (Mammalia, Proboscidea) from the Miocene-Pliocene of North America.- New Mex. Mus. Nat. Hist. Sci. Bull. 44: 71-87. Tetrabelodon brevidens Cope, 1889 Dibelodon praecursor Cope, 1893 Rhynchotherium rectidens Osborn, 1923 Aybelodon hondurensis Frick, 1933 Blickotherium blicki Frick, 1933 Rhynchotherium anguirivale Osborn, 1936 Phylogenetic position according to Mothé et al. (2016)
Life history Rhynchotherium appears to have travelled in large mixed-age herds. At least two Rhynchotherium death assemblages, dubbed "elephant graveyards", are known from opposite sides of the continent, in California and Florida. The Florida site contains 3,000 individual fossils representing 38 individuals, including at least one complete adult and seven complete juveniles. The site appears to represent an area where Rhynchotherium herds repeatedly became trapped and died, potentially around the curve of a river that they periodically crossed. Some individuals may have also been washed into the site from upstream.
Palaeoecology Although traditionally thought to be a browser, analysis of enamel δ13C of Rhynchotherium teeth from the Bone Valley deposits of Florida dating to the Early Pliocene suggests that they fed almost exclusively on C4 grasses.
References